The VRay Render Mask was added back in Vray 3.0 I think, you can do include/excludes, choose to only render your selected object or put an image mask in to render this.
Back then, I wrote this tool, which I completely forgot about until today which allows you to basically draw out multiple render regions and set this up as Render Mask.
It has a fairly simple but nifty UI with some clever bits of power, like being able to use MME and adding the selected objects… (using a very dirty hack). Most of the time I think most people will just draw out areas. Overlapping areas don’t get rendered twice so you can draw some fine detailed areas to re-render if you want, but always give yourself a little bit of bleed as there’s a small precision loss in the scaling.
Find it on…